Radiographic method for the detection of residual core material comprising the use of radiographically detectable powder

ABSTRACT

Method of detecting the presence of core material in a cored casting comprising injecting a radiographically detectable powder into the passages of the hollow casting, vibrating the casting to aid in distributing the powder throughout the passages, exposing the treated casting to penetrating radiation, and then inspecting the casting.

United States Patent lnventors Appl. No. Filed Patented Assignee RADIOGRAPHIC METHOD FOR THE DETECTION OF RESIDUAL CORE MATERIAL COMPRISING THE USE OF RADIOGRAPHICALLY DETECTABLE POWDER 1 1 Claims, 3 Drawing Figs.

X-ray Examination of Steel Castings by H. H, Lester, Chemical and Metallurgical Engineering, Feb. 7, 1923, reprinted in Materials Evaluation,.lan. i969, pgs. 3 to 10 Primary ExaminerArchie R. Borchelt Attorney-Hill, Sherman, Meroni, Gross & Simpson ABSTRACT: Method of detecting the presence of core material in a cored casting comprising injecting a radiographi- 0.8. CI 250/65, detectable Powder imo the Passages of the hollow cast 25 /7 250 33 ing, vibrating the casting to aid in distributing the powder Cl 0 23 0 throughout the passages, exposing the treated casting to Field of Search 250/65, 71, Penetrating radiation. and inspecting the casting RADIOGRAPHIC METHOD FOR THE DETECTION OF RESIDUAL CORE MATERIAL COMPRISING THE USE OF RADIOGRAPHICALLY DETECTABLE POWDER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is in the field of casting metals and more specifically, in the field of detecting the presence and the location of occlusions in hollow passages present in castings by radiographic analysis.

2. Description of the Prior Art Many precision parts having complex internal passages are being manufactured by means of precision investment casting processes. Where the casting is composed of a very high melting metal or alloy, the passages are formed by providing a siliceous core capable of withstanding the temperature of the molten metal without buckling or deformation. After the casting has been completed, the problem remains of removing the core from within the body of the casting to provide the open passages. Generally, a leaching agent such as a concentrated solution of hydrofluoric acid or sodium hydroxide, or molten caustic is used for this purpose. It is important that the removal of the siliceous core and the leaching agent itself be complete as otherwise there would be obstructions in the 5 passages preventing the free flow of cooling medium which the presence of the passages was designed to facilitate.

It is, of course, possible to determine the presence of foreign bodies such as inclusions in the passages by gravimetric means, but it is also readily apparent that such gravimetric analyses are extremely limited in accuracy, particularly where the residual core inclusions are small and the passages have narrow openings. It is also possible to detect the presence of core materials by selective neutron absorption radiography where core materials are doped with neutron absorbers but this procedure is very expensive.

In Dalton U.S. Pat. No. 2,812,562, issued Nov. 12, 1957, there is described a method of detecting glass core residues by means of using a glass core which is coated with a material having a high-radiographic density. After treatment of the glass core with a leaching agent, a radiographic inspection is made, relying on the contrast provided between the coating material and the metal of the casting to provide an indication of undissolved coatings, and inferentially, of undissolved glass core.

SUMMARY OF THE INVENTION The present invention involves the method for the location of obstructions in internally cored castings wherein a radio- 5 The actual inspection can then be carried out by several means which may be by fluoroscope, electronic image enhancement, or by providing a permanent record on radiation sensitive film.

Other objects, features and advantages of the invention will 1 be readily apparent from the following description of a preferred embodiment thereof, taken in conjunction with the accompanying drawing, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosures, and in which:

FIG. 1 is a schematic representation of one system for inspecting a casting for the presence of obstructions, using a radiation sensitive film to provide a permanent record:

FIG. 2 is a cross-sectional view of the assembly shown in FIG. 1, taken substantially along the line II-II; and

FIG. 3 is a greatly enlarged fragmentary view of a passage partly obstructed by a siliceous core DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides'for the location of obstructions in cored castings by virtue of difi'erences in reaction to penetrating radiation between the metal of the casting, the powder and the material of the core which is normally a nonmetal. Differences in reactivity to penetrating radiation are 30 measured by means of attenuation coefficients from the well known Lambert-Bouquer Law which can be stated as follows: l =l =I eI/F P I where 1,, is the initial intensity of the incident radiation 1, is the intensity after passage through an object of thickness x p is the density p. is the linear attenuation coefiicient ,u./p is the mass attenuation coefficient which is a function of the atomic number of the attenuating element and the wave length of the incident radiation.

For the purposes of the present invention, the attenuation coefficients of the casting metal and the powder used to fill the passages are substantially matched. To do this, we select a powder of known mass attenuation coefficient and packing fraction for vibratory compaction such that the effective radiation attenuation is within 30 percent, plus or minus, of the mass attenuation coefficient of the metal of the casting. The packing fraction is the relationship which the apparent density of the powder in its packed condition bears to the theoretical density of the metal of the powder.

The following table lists the mass attenuation coefficients for various metals at particular energies:

Muss attenuation coefficients. cmfi/gni.

Radiation, Al, Ti. Fe, C0, Ni, Cu, Mo. Tu. W, energy (m.e.v.) 2:13 2:22 2:26 2:27 2:28 2:29 2:42 2:73 Z: 74

in the first step of the process, the internal passages of the part to be inspected are first filled with a finely divided powder having attenuation characteristics approximating those of the metal casting, as previously noted, but less or more dense powders may be used with reduced sensitivity. Then, the part is vibrated at sonic frequencies ranging from about 10 to 20,000 Hertz. to pack the powder and allow trapped air to escape. During vibration, any additional powder necessary to completely fill the passages is added.

The vibration frequency of the part should be controlled to insure proper filling. Our experience has indicated that the flow of powder is readily facilitated by control of the peak acceleration imparted to the powder particles by the vibrating part. The peak acceleration is calculated as follows:

Peak Acceleration 41r2,2,,

where f is the frequency and d is the displacement amplitude.

We have found that the lowest useful peak acceleration appears to be about 20 meters per second squared, while the highest permissible peak acceleration value depends on the fatigue or yield stress limits of a particular cast part.

The proper particle size and shape is also important for achieving best results. Broadly speaking, the particle size range should be in the order of 40 to +625 Tyler mesh, although particles smaller than 625 mesh can be tolerated if they represent less than 10 percent of the volume. The most desirable range is l to +400 Tyler mesh. Although acicular powders are undesirable because of their poor flow properties, powders which are only roughly spherical in shape can be used to advantage in this process.

Once the passages have been filled and the metal powder subjected to a tamping action by the vibration of the casting, the casting is ready for inspection a illustrated in FIGS. 1 and 2 of the drawings.

The two figures of the drawings illustrate schematically the inspection method as it is applied to the inspection of a turbine blade having an airfoil portion 11 and a root portion 12. Located within the airfoil portion II are a plurality of hollow passages 13 which have been formed by casting a high temperature alloy or superalloy about a ceramic-type core. pbn completion of the casting operation, the ceramic core is treated with caustic or hydrofluoric acid to remove as much of the core as possible. To check the completeness of core removal, the passages 13 are filled with a radiographically detectable powder 14 and subjected to vibration as previously described. Then, the turbine blade 10 is exposed to penetrating radiation whose field is represented by the arrows 15 in FIGS. 1 and 2. Behind the blade 10 there is exposed a radiation sensitive detector such as a film 16. It should be evident, of course, that direct fluoroscopic examination, or electronic image enhancement can also be employed if desired.

When the powder attenuation coefficient and the packing of the powder in the passages provide radiation characteristics in the powder which approximate that of the casting, the part appears to be a solid piece of cast metal with little or no evidence of internal passages, except where residual materials 17 (see FIG. 3) have blocked the passages and partially or completely excluded the powder. in those areas, as illustrated in FIG. 2 of the drawings, indications 18 will appear on the photograph to identify the location of the obstructions in the passageways.

The process of the present invention, of course, is applicable to many different varieties of metals, and to any shape of casting. We have achieved particularly good results by using this process of inspection with cast nickel or cobalt superalloys and using molybdenum powder having a packing fraction of about 30 percent to provide the indications. Higher and lower attenuating powders such as tungsten and iron can be used but are less effective. In this latter case the indications of occlusions may not clearly discernible and unambiguous.

The process of the present invention is considerably less expensive than selective neutron absorption radiography since standard X-ray or gamma-ray sources can be used instead of a nuclear reactor. Such standard sources may produce energies of about 50 to 250 Kev. for ordinary size parts, but higher energy sources can be used for the examination of larger parts. The process has the further advantage that the indicating powder may be reclaimed and reused. It provides a method which resolves a much smaller residual inclusion than other methods heretofore proposed. With the use of powder, no surface tension effects occur and trapped air can escape through the open porosity of the powder.

We claim as our invention:

1. The method of detecting presence of core material in a casting having internal passages which comprises injecting a radiographically detectable powder into said passages, vibrating the casting to aid in distributing said powder throughout said passages, exposing the thus treated casting to penetrating radiation, and inspecting the casting.

2. The method of claim 1 in which the product of the mass attenuation coefficient and the packing fraction of said powder is within 30% of the mass attenuation coefiicient of the casting.

3. The method of claim 1 in which the powder size is in the range from 40 to +625 Tyler mesh.

4. The method of claim 1 in which the powder size is in the range from l 00 to +400 Tyler mesh.

5. The method of claim 1 in which the casting is vibrated at a peak acceleration of at least 20 meters per second squared.

6. The method of claim 1 in which said casting is composed of an alloy selected from the group consisting of nickel and cobalt base alloys and said powder is molybdenum.

7. The method of claim 5 in which the frequency of vibration is in the range from 10 to 20,000 Hertz.

8. The method of detecting the presence of core material in a casting having internal passages which comprises injecting a radiographically detectable powder into said passages, the product of the mass attenuation coefficient and the packing fraction of said powder being within 30 percent of the mass attenuation coefficient of the metal of the casting, said powder having a particle size in the range from 40 to +625 Tyler mesh, vibrating the casting at a frequency of from 10 to 20,000 Hertz to achieve a peak acceleration of at least 20 meters per second squared in said casting, exposing the thus treated casting to penetrating radiation, and measuring the radiation passing through the casting.

9. The method of claim 8 in which the measurement is carried out fluoroscopically.

10. The method of claim 8 in which the measurement is carried out photographically.

11. The method of claim 8 in which the measurement is carried out by electronic image enhancement. 

1. The method of detecting presence of core material in a casting having internal passages which comprises injecting a radiographically detectable powder into said passages, vibrating the casting to aid in distributing said powder throughout said passages, exposing the thus treated casting to penetrating radiation, and inspecting the casTing.
 2. The method of claim 1 in which the product of the mass attenuation coefficient and the packing fraction of said powder is within 30% of the mass attenuation coefficient of the casting.
 3. The method of claim 1 in which the powder size is in the range from -40 to 625 Tyler mesh.
 4. The method of claim 1 in which the powder size is in the range from -100 to +400 Tyler mesh.
 5. The method of claim 1 in which the casting is vibrated at a peak acceleration of at least 20 meters per second squared.
 6. The method of claim 1 in which said casting is composed of an alloy selected from the group consisting of nickel and cobalt base alloys and said powder is molybdenum.
 7. The method of claim 5 in which the frequency of vibration is in the range from 10 to 20,000 Hertz.
 8. The method of detecting the presence of core material in a casting having internal passages which comprises injecting a radiographically detectable powder into said passages, the product of the mass attenuation coefficient and the packing fraction of said powder being within 30 percent of the mass attenuation coefficient of the metal of the casting, said powder having a particle size in the range from -40 to +625 Tyler mesh, vibrating the casting at a frequency of from 10 to 20,000 Hertz to achieve a peak acceleration of at least 20 meters per second squared in said casting, exposing the thus treated casting to penetrating radiation, and measuring the radiation passing through the casting.
 9. The method of claim 8 in which the measurement is carried out fluoroscopically.
 10. The method of claim 8 in which the measurement is carried out photographically.
 11. The method of claim 8 in which the measurement is carried out by electronic image enhancement. 